Outer hair cells amplify sound waves more than fortyfold
Hearing is not a passive microphone. After the outer ear and middle-ear bones raise pressure, cochlear outer hair cells lengthen and shorten, boosting traveling-wave amplitudes over forty times before inner hair cells turn vibration into nerve signals.
The auditory system pairs the ears with brain pathways for the sense of hearing. Pinna folds of cartilage reflect and attenuate waves to cue direction; the canal boosts roughly 3–12 kHz sounds toward the tympanic membrane. Malleus, incus, and stapes—the hammer, anvil, and stirrup—lever low-pressure eardrum motion into higher pressure at the oval window, about a twentyfold gain, because the cochlea beyond is fluid-filled rather than air-filled. The stapedius reflex stiffens the chain against damaging loud energy while information is still in wave form.
Inside, perilymph fills vestibular and tympanic ducts while endolymph fills the smaller cochlear duct at a different ion mix and voltage. Basilar membrane stiffness grades from base (narrow, high frequencies) to apex (wide, low), paralleled by the tectorial membrane. Outer hair cells, three to four times more numerous than inner ones, release prestin-driven length changes that boost traveling-wave amplitudes over fortyfold; inner hair cells are the true receptors, each contacted by many afferent fibers. Bundles of 100–200 stereocilia linked by tip links of cadherin-23 and protocadherin-15 tug open ion channels as the tectorial membrane shears.
Most cochlear-nucleus fibers cross in the trapezoid body to aid localization. Bushy, stellate, and octopus cells extract timing, spectra, and temporal precision before the superior olivary complex compares left-right delay and level. Ascending tracts climb the lateral lemniscus and inferior colliculus—where visual, cerebellar, and startle circuits mingle—then the medial geniculate thalamus to auditory cortex, which maps frequency, harmony, and source. Right cortex weighs tonality; left weighs fine sequences. More than thirty thousand cochlear nerve fibers, each with a best frequency, begin that journey from the organ of Corti.
Source: Auditory system
